This invention relates in general to electronic assemblies and testing thereof. In particular, the invention relates to a method of manufacture of protruding, controlled aspect ratio and shape contacts for uses in interconnections of assemblies and testing thereof.
Interconnections which involve protruding electrical contacts are used extensively in packaging of electronics. Pin grid array packages, both plastic and ceramic, housing a variety of semiconductors, use area arrays of pins as interconnect contacts for connection to circuit boards. Pins can be attached to their receiving package conductors by use of a variety of methods. For ceramic packages, pins are inserted into non-reacting brazing fixtures and are then gang-brazed to corresponding conductive terminals on the package. This approach is characterized by significant non-recurring engineering costs and lead times involved in production of the brazing fixture. Plastic pin grid array packages most commonly use pins which are inserted into metallized through holes in a circuit board, while the dimensions of pins and the holes normally chosen to facilitate good contact between the walls of the pins and the coating of the holes. This approach has a disadvantage in that the coated holes and the pins block some circuit routing channels within the circuit board, thus forcing either use of narrow circuit traces, or increase in circuit board area, either of which results in increased costs.
Permanent connection of the pin grid array packages to circuit boards often is accomplished by inserting pins through corresponding holes in a circuit board, the pins protruding to a predetermined length beyond the circuit board. A resulting assembly then is passed through a wave soldering machine, and the pin grid array thus is soldered to the circuit board. Alternatively, a pin grid array can be inserted into a low insertion force or zero insertion force socket for a demountable assembly. Such a socket, in its turn, normally is connected permanently to a board.
A current trend in interconnections is toward face-to-face surface mounting of components to boards and semiconductor chips to substrates. This approach is best accomplished with protruding contact structures on top of (or otherwise protruding from) contact carrying conductive terminals or traces. Conductive terminal arrangements on facing components and substrates are increasingly being made of the area array type, as this allows for larger contact-to-contact separation as compared with components characterized by peripheral arrangement of interconnection contacts.
Pins attached to either ceramic or plastic packages according to the traditional methods are, in general, not appropriate for mounting to patterns of surface contacts on circuit boards, due to pin length variation. For surface mounting, the pins would have to be planarized, which represents an additional expensive step subsequent to pin assembly. In addition, there is a significant cost penalty associated with production of pin-carrying packages with pin-to-pin separations of 50 mils, or lower.
There is currently an increasing need for a low cost method of attaching protruding contacts from conductive terminals, arising from proliferation of surface mountable area array contact packages. Stand-off height of protruding contacts is particularly important when coefficients of thermal expansion of components and of circuit board materials differ significantly. The same is true for attachment of un-packaged semiconductor chips to interconnection substrates. These expansive concerns call for a low cost, high volume method of manufacturing protruding, controlled aspect ratio or shape electrical contacts on top of (or otherwise protruding from) contact carrying conductive terminals, on top of any device or circuit bearing substrate, board material or component, and its applications to surface mount interconnections of devices, components and substrates.
U.S. Pat. Nos. 5,189,507, 5,095,187 and 4,955,523 disclose a method of manufacturing of controlled height protruding contacts in a shape of wires for direct soldering to a mating substrate. The wires are bonded to terminals without use of any material other than that of wire and the terminals, using ultrasonic wirebonding methods and equipment, which comprises a standard industry technique for interconnecting semiconductor chips to packages. The patents also describe a bonding head which incorporates a wire weakening means for controlling length of free standing severed wires. Vertically free standing wires present a handling problem during assembly, which is addressed in the patents by providing for polymer encapsulation of bonds between the wires and terminals. The polymer coating, which is optional, also compensates for another disadvantage of the approach, namely weak points along the wire, typically the point of contact between the wire and terminal metallization, and in case of ball bonding, in a heat effected zone of the wire just above impression of a bonding capillary. While these patents provide for controlled height contacts, and discuss 2d to 20d aspect ratios, in practice they do not assume controlled aspect ratios for all kinds of protruding contacts which are required in various applications. For instance, standard, high speed wirebonding equipment could not handle a 30 mil diameter wire. Therefore, according to these inventions, a 30 mil diameter, 100 mil high contact could only be produced on lower throughput specialized equipment, at higher cost. In addition, a gold wire as described in a preferred embodiment, would have a problem of dissolving in solder during a soldering cycle, which causes long term reliability problems with solder joints. Similarly, direct soldering of copper contacts would in many cases result in undesirable reaction between copper and solder at elevated temperatures. While nickel metal is the material of choice for solder joint reliability, nickel wire can not be used for ultrasonic wirebonding to metal terminals due to its high mechanical strength and passivating, oxide forming properties. Chemical, physical and mechanical properties, as well as permissible dimensions and shapes of the protuberant contacts produced according to this invention are limited to the capabilities and materials choices compatible with known wire bonding techniques.
U.S. Pat. No. 3.373,481 describes a method of interconnecting semiconductor components on substrates by means of dissolving protruding gold projections on the components in solder masses formed on the substrate terminals. The gold projections are formed by compression and extrusion of gold balls against the terminals. This approach is incapable of producing high aspect ratio protruding contacts because of limitations of the extrusion method. In addition, dissolution of gold in solder, as taught by this approach creates a problem due to reliability concerns. The method also limits selection of contact material to easily extrudable metals, like gold.
There are several methods in the prior art for controlled elongation of solder masses between a component and a substrate. The goal is to create a column-like solder shape, preferably an hourglass shape, in order to achieve increased resistance to thermal cycling. To that end, U.S. Pat. No. 5,148,968 discloses a web-like device which upon heating during solder reflow stretches the solder connections, forming such hourglass shaped solder columns. Aspect ratios of the columns are determined by the mass of solder in the joint, dimensions of the solder wettable terminals on the substrate and the component, and by the characteristics of the stretch device. This method is only limited to contact materials which are reflowed during the assembly, and requires external hardware for forming the contact shape, which adds cost of the hardware and increases the process complexity.
U.S. Pat. No. 4,878,611 teaches formation of controlled geometry solder joints by applying controlled volumes of solder to a semiconductor package and a substrate, bringing solder masses in contact, and reflowing the solder masses while maintaining controlled separation between the component and the substrate through mechanical means. This approach also requires hardware means for maintaining longitudinal dimension of the contacts, and does not lend itself to standard practices of surface mount soldering, in which industry already has made substantial time and capital investments.
In the same spirit, U.S. Pat. No. 4,545,610 discloses a process for forming elongated solder joints between a plurality of solder wettable terminals on a semiconductor component and a substrate by means of solder extenders, which transform the shape of the solder joints into uniform hourglass shapes during a solder reflow step. This approach requires additional, non-standard processing of either a silicon device, or on a substrate, which includes attachment of reflowable solder extenders, and nonreflowable means of maintaining vertical spacing between the silicon device and the substrate.
U.S. Pat. No. 5,154,341 discloses the use of a spacer bump which is composed of a solder alloy which does not melt at the temperature of component soldering. A eutectic solder microball is placed on a contact, and upon the reflow the reflowable solder encases the non-reflowable spacer, and produces an hourglass shaped joint. The aspect ratios of the joint contacts are controlled by dimensions of the spacer.
U.S. Pat. No. 4,332,341 teaches solid state bonding of solder preforms to components, substrates or both, for further joining. Resulting protruding solder contacts either collapse during soldering if they consist of eutectic solder, or do not reflow at all, when they consist of higher melting temperature solder. In the latter case, a component would not have the benefit of a self-alignment effect as the pool of solder is confined to the vicinity of the terminals, and a main portion of the joint which controls aspect ratio, does not melt.
U.S. Pat. No. 4,914,814 discloses a process for fabricating solder columns through use of a mold with an array of pin holes, which are filled with solder, brought into contact with terminals on a component, and bonded by melting the solder which wets the terminals. The component with columns is then bonded to a substrate through reflow in a solder with lower melting temperature than the solder of the columns. This approach requires generating a mold for each solder column array pattern, commonly involving undesirable non-recurring engineering expenses and associated increase in delivery times.
U.S. Pat. No. 3,509,270 discloses a printed circuit assembly which is interconnected with a dielectric carrier that has a plurality of spring elements positioned in its apertures, the springs are optionally encased in a solder material to facilitate permanent electrical contact between circuit elements. This approach requires a custom interposer pattern manufactured for each application, while solder coated springs would have to be placed individually inside the apertures. Additionally, soldering usually requires flux, while the interposer material makes it difficult to clean flux after the reflow process.
U.S. Pat. Nos. 4,664,309 and 4,705,205 disclose a device and a method for interconnection with reinforced solder preforms which use a retaining member provided with apertures. The retaining member is optionally dissolved to leave resilient interconnect structures. The solder columns maintain their shape in the molten state, supported by particles, filaments, spiral metallic wire or tape. 20 to 80% by weight of filler material is specified. This approach, as several of the above described approaches, requires a custom made retaining member for every interconnect application, and therefore requires an additional non-recurring engineering expense and increased delivery time for every production order.
U.S. Pat. No. 4,642,889 teaches use of an interposer with a plurality of interconnect areas, interconnect areas comprise wire means surrounded with solder, and incorporate a soldering flux material. Upon heating the solder reflows and connects to the terminals of the mating components and boards, while the wires in the middle of each solder joint insure a column-like joint shape. The interposer preferably is dissolved away. While providing means for controlled aspect ratio interconnect joints, this approach also requires use of a custom manufactured interposer and inevitable non-recurring engineering expenses and increased delivery times. In addition, when improved alignment of the interconnect to the terminals is required, as terminal-to-terminal distances decrease, this approach suffers from heat and environmental effects on interposer material, causing it to distort or change dimension during processing steps, which makes the alignment difficult, and limits this approach to relatively coarse terminal-to-terminal pitch applications.
An elongated protruding solder contact is taught according to U.S. Pat. No. 5,130,779, by sequentially encapsulating solder deposits with a barrier metal. This approach insures that a staged solder deposit does not collapse upon reflow, and a controlled aspect ratio solder contact is thus formed. In order for the barrier metal to be effective, the walls of sequentially deposited solder masses must have a slope. Deposition of such a structure of solder is a lengthy and expensive process.
A method of attaching highly compliant leads to an array matrix of contacts on a semiconductor package is disclosed in U.S. Pat. No. 4,751,199. An array of leads is manufactured according to the required pattern with temporary tab structures tying the leads together. Compliant leads are gang bonded to terminals on the semiconductor package, and tabs are removed leaving compliant protruding contact leads, ready for attachment to a substrate. This approach, like the interposer techniques described above, requires specific tooling for every new package with a distinct pattern of the array of contacts.
One object of the present invention is to provide a high volume, low cost manufacturing process for production of precise shape and geometry protuberant conductive contacts on a wide variety of electronic components, the contacts having a controlled set of physical, metallurgical and mechanical properties, bulk and surface. Such protuberant contacts can be employed to satisfy electrical, thermal or geometrical requirements in various aspects of electronic interconnection applications. An inventive, multiple stage process is provided, according to which, first, wire stems are bonded to contact carrying terminals, the stems being shaped in three dimensional space, to define skeletons of the resulting contacts. The conductive contacts are then completed through at least one deposition step of a coating which envelopes or jackets the wire skeletons and the terminals. The common coating not only helps to xe2x80x9canchorxe2x80x9d protruding contacts to the terminals, but also provides for characteristics of the protuberant contacts with respect to long term stability of their engagement contact with mating electronic component, including but not limited to interconnection substrates, semiconductor devices, interconnect or test sockets. The coating also determines soldering assembly characteristics, as well as long term effects from contact with solder. The coating, along with the wire material properties, determines the mechanical characteristics of the resulting protuberant contact. The wire serves as a xe2x80x9cskeletonxe2x80x9d of a resulting protruding contact, and the coating serves as its xe2x80x9cmusclexe2x80x9d.
The wire skeleton can be bonded employing high productivity, highly automated ultrasonic, thermosonic and thermocompression wirebonding equipment (hereinafter referred to as ultrasonic wirebonding equipment). The wirebonding equipment can be organized for ball bonding type ultrasonic wirebonding process, typically used for gold or copper wirebonding, but modifiable for use with other types of single phase or noble metal coated wires. Wire skeletons consisting of free standing single or multiple wire stems can be produced. As required by the shape and dimensions of the resulting conductive contacts, the stems can be standing normal to a terminal or at an angle thereto, or can be formed in three dimensional space. Alternatively, wire skeletons produced using an ultrasonic ball bonder can be produced by forming a traditional wire-bonding loop, which originates and terminates on a contact carrying terminal, or which originates on the contact carrying terminal and terminates on a sacrificial conductor outside the terminal area. Then the sacrificial conductor is selectively dissolved away after completion of the contact processing. Optionally, a wedge bonding type ultrasonic wirebonder can be employed, in which case controlled shape loops of wire are formed with both ends of a loop on the same terminal, or they are formed by originating the loops within the area of the terminal and terminating or severing the loops outside the terminals. Both types of wirebonders are available commercially from a variety of suppliers, and are highly automated, with the bonding speeds exceeding 10 wires per second.
An additional object of the present invention is to produce array patterns of contacts for unique component designs, without incurring costs and delays associated with production of unique tooling. This object is achieved through use of a sequential wirebonding process for attaching of the wire skeletons to the terminals, and then overcoating them in a single, non-pattern-specific step. Location targeting and geometric characteristics are entered into an electronic control system of the wirebonding equipment with a specific set of commands. A program controls all aspects of wire forming and shaping process, as well as bond characteristics. Production of the multiple protruding contacts does not require time-limiting and costly production of molds, masks or the like, which otherwise would be specific to every incoming order. This object is exemplified by application of the present invention to production of integrated pin-shaped contacts for ceramic pin grid array packages. This is achieved by bonding straight vertical, single stem skeletons, perpendicular to the surface of terminals, and overcoating them with a conductive structural metal or alloy, consisting substantially of nickel, copper, cobalt, iron or alloys thereof, with the thickness of the deposit determined by the required pin diameter value. This can also be accomplished with equal ease on plastic pin carrying substrates.
Yet another object of the present invention is to create protruding, tower-like solder contacts which substantially maintain a well controlled aspect ratio even when the solder is molten. This object is achieved through first bonding a wire skeleton which will define the final shape and aspect ratio of a solder contact, then optionally overcoating it with a barrier layer which prevents long term reaction between the solder and the wire, and finally depositing a mass of solder which wets the skeleton with a barrier, and clings onto the skeleton even after multiple reflows.
Yet another object of the present invention is to create controlled shape resilient contacts on top of (or otherwise protruding from) contact carrying terminals arranged in various patterns, including arrays. This object is achieved by overcoating the wire skeleton with at least one coating having a predetermined combination of thickness, yield strength and elastic modulus to insure predetermined force-to-deflection characteristics of resulting spring contacts.
Yet another object of the present invention is to produce protruding contacts on top of (or otherwise protruding from) a multitude of contact-carrying terminals, where the terminals have varying vertical coordinates, corresponding to an origin point of the protruding contacts, while uppermost points of the protruding contacts extend to a vertical coordinate which is substantially identical, within a permissible tolerance level for the component or substrate which is to be subsequently connected to the contacts. In a different embodiment related to this object, the terminals can originate on various electronic components, while the protuberant contacts extend to a substantially identical vertical coordinate. This object is achieved by controlling a wire severing step, by always severing the wire at a predetermined vertical coordinate. Overcoatings then follow the skeleton wire shape and geometry, yielding a contact having a controlled vertical coordinate level, regardless of the Z-axis point of origination of the contact, e.g. plane of the terminal. Some of the terminals having varying Z-coordinates and overlying the component, can optionally and additionally overlie single or multiple electronic devices, placed on the electronic component. In another embodiment related to this object of the present invention, the vertical coordinate of the tips of the protuberant contacts can be controlled by the software algorithm of the appropriately arranged automated wirebonding equipment.